1. Field of the Invention
The present invention relates to a silver halide photographic photosensitive material and an image forming method utilizing the same, and more particularly to a silver halide photographic photosensitive material suitable for rapid processing, capable of showing rapid development progression and providing high-contrast gradation even in digital exposure such as laser-scan exposure, and an image forming method utilizing the same.
2. Description of the Related Art
Recently, digital technology has shown remarkable pervasiveness even in the field of color printing utilizing a color photographic paper, and, for example, a digital exposure method based on laser-scan exposure is showing a drastic increase in comparison with a prior analog exposure method in which a print is printed with a color printer from a developed color negative film. Such a digital exposure method has an advantage of obtaining high image quality by image processing, and therefore plays an extremely important role in improving the quality of the color print utilizing the color photographic paper. In view of the rapid spread of digital cameras, it is also an important factor for such a printing method that a high-quality color print can be easily obtained from electronic recording media of such digital cameras, and these factors are anticipated to bring about a further expansion of such a printing method.
On the other hand, various other printing technologies such as ink jet recording, sublimation transfer recording and color electrophotography have shown progress and, being praised for photographic quality, are also recognized as color printing methods. Among these methods, the digital exposure method utilizing the color photographic paper is characterized by high image quality, high productivity and high image durability, and is desired to more simply and more inexpensively provide photographs of a higher quality by fully exploiting such characteristics. The superiority of the color print utilizing the color photographic printing paper will be further enhanced by making it possible, after a recording medium of a digital camera is received at a shop, to prepare a high-quality print within a short time on the order of several minutes and deliver the print to the customer on the spot.
In the color photographic paper, in order to meet requirements for rapid development, a silver halide emulsion with a high content of silver chloride is employed. It is generally known that rapid developability can be further improved by employing a silver halide emulsion of a smaller grain size. However, it has been found that a reduction in grain size in an emulsion with a high silver chloride content tends to cause, particularly in high illumination intensity exposure such as laser-scan exposure, a variation in density resulting from a fluctuation in development factors in a gradation exposed area, thereby making it impossible to obtain the advantage of rapid developing in significant manner.
Iridium doping is known for improving reciprocity failure at a high illumination intensity in the silver chloride emulsion and obtaining a high contrast gradation even under a high illumination intensity. However the silver chloride emulsion doped with iridium is known to result in latent image sensitization within a short time after exposure, and, for example, Japanese Patent Application Publication (JP-B) No. 7-34013 discloses a method of avoiding latent image sensitization by forming a localized phase of a high silver bromide content and executing iridium doping therein. A silver halide emulsion prepared according to such a method shows a high sensitivity and a high contrast and is free from latent image sensitization even under exposure of a relatively high illumination intensity such as that of about 1/100 seconds, but it has been found difficult to obtain a high contrast gradation when attempting to maintain a high sensitivity to an ultra-high illumination intensity exposure of 1 microsecond which is required in the digital exposure method based on laser-scan exposure. U.S. Pat. No. 5,691,119 discloses a method of obtaining a high contrast gradation under a high illumination intensity in the preparation of an emulsion having a localized phase with a high silver bromide content, but such a method is not sufficiently effective and has a drawback in that performance is unstable in the repeated preparations.
U.S. Pat. Nos. 5,783,373 and 5,783,378 disclose methods of reducing reciprocity failure and obtaining a high contrast gradation by employing at least three dopants. However, a high contrast gradation is realized by the use of a dopant having a contrast increasing effect with a desensitizing effect, and is therefore incompatible in principle with obtaining a higher sensitivity.
U.S. Pat. Nos. 5,726,005 and 5,736,310 disclose obtaining a high sensitivity and a reduced reciprocity failure under a high illumination intensity by using an emulsion containing iodine with a density maximum at a sub-surface of the emulsion grains having high silver chloride content. Also, EP 0,928,988A discloses, in an example thereof, obtaining an emulsion having improved reciprocity failure, and superior temperature dependence and pressure resistance at exposure by including a specific compound in grains in which an I-band is formed at a point of 93% grain formation and which has a side length of 0.218 μm, i.e. a sphere-equivalent diameter of about 0.27 μm. However, though such a silver halide emulsion of a high silver chloride content having a small grain size as disclosed in these references certainly shows a high sensitivity under exposure at a higher illumination intensity, it has been found to show a quite low contrast gradation under an ultra-high illumination intensity exposure such as laser-scan exposure, thus being unsuitable for digital exposure which is limited in the dynamic range of the light amount, and also to have a drawback in that a latent image is poor in stability over a period of several seconds to several tens of seconds after exposure.
Japanese Patent Application Laid Open (JP-A) Nos. 58-95736, 58-108533, 60-222844, 60-222845, 62-253143, 62-253144, 62-253166, 62-254139, 63-46440, 63-46441 and 63-89840, and U.S. Pat. Nos. 4,820,624, 4,865,962, 5,399,475 and 5,284,743 disclose that a high sensitivity can be obtained by including a phase of a high silver bromide content in a localized manner, in various forms, in an emulsion of a high silver chloride content. However these references do not describe a contrast increasing effect under ultra-high illumination intensity exposure such as laser-scan exposure.
Performing selenium sensitization to attain a high sensitivity of an emulsion having a high silver chloride content is well known. This is described, for example, in JP-A Nos. 4-328541, 4-335338, 4-335346, 5-66513, 7-140579 and 8-171168. However, while chemical sensitization with a selenium compound may produce a strong sensitizing effect, it also tends to cause strong fog and to lead to soft gradation. Many patents which have been disclosed so far have attempted to resolve such drawbacks, but results still remain disappointing. Furthermore, while performing chemical sensitization utilizing a gold compound in combination with a selenium compound produces a significantly improved sensitivity, at the same time it tends to cause a further increase in fog and heighten the risk of soft gradation.
A silver chloride emulsion sensitized with a selenium compound is disclosed in JP-A No. 7-140579. The silver chloride emulsion is reported to have a high sensitivity and a low fog when exposed for 1/10 seconds, but use in a color photographic paper it has an excessively high fog density. In addition, the reference does not teach anything about rapid processability under a high illumination intensity or about securing a high-contrast gradation.
JP-A No. 5-66513 discloses an emulsion formed by selenium sensitization of silver chloride grains of at least 0.35 μm and which is particularly useful for an yellow layer of a color print film. However, it does not teach important features for a color photographic paper, rapid processability under a high illumination intensity and high-contrast gradation.
A silver chloride emulsion utilizing colloidal selenium is disclosed, for example, in JP-B No. 44-15748, but the reference does not teach the size of the grains employed, or an exposure time utilized for evaluating a sensitivity thereof. The reference merely describes the application of colloidal selenium to the silver chloride emulsion, but discloses nothing on important features of the invention, rapid processability under a high illumination intensity and a high-contrast gradation.
As explained in the foregoing, various technologies have been disclosed to rectify reciprocity failure under a high exposure intensity and to obtain a high contrast gradation in a high silver chloride emulsion, but such technologies have drawbacks. In particular, if a grain size of these emulsions is reduced in order to achieve ultra rapid developability, a fluctuation in density is often caused, particularly in high-intensity exposure such as laser-scan exposure, resulting from a fluctuation in process factors in a gradation exposed area.